Method of making light-duty electrical contacts composed of silver and ruthenium dioxide

ABSTRACT

A light-duty electrical contact material which consists of a mixture of silver and ruthenium dioxide, the ruthenium dioxide content being in the range 0.1 to 13.0 atomic percent. For reasons of economy, the preferred ruthenium dioxide content is 1.3 percent. The material is produced by powder metallurgical techniques from fine, irregular silver powder and preferably ultra fine ruthenium metal powder. Fine ruthenium dioxide powder can be utilized in place of the ruthenium metal powder.

waited States Patent 1 Davies Get. 15, 1974 [5 METHOD OF MAKING LIGHT-DUTY 2,664,618 1/1954 Kleis 75/206 ELECTMCAL CONTACTS CQMPOSED 0F 2,894,839 7/1959 Matsukawa 75/206 SILVER AND RUTHENIUM DIOXIDE [75] Inventor: Terrence Ardern Davies,

Northampton, England [73] Assignee: Square D. Company, Park Ridge, 111. [22] Filed: June 25, 1973 [21] Appl. No.: 373,053

Related US. Application Data [62] Division of Ser. No. 191,564, Oct. 21, 1971, Pat. No.

[30] Foreign Application Priority Data Oct. 21, 1970 Great Britain 49960/70 [52] US. Cl. 75/206, 75/224 [51] Int. Cl B22f l/00 [58] Field of Search 75/206, 224

[56] References Cited UNITED STATES PATENTS 2,486,341 10/1959 Stumbock 75/206 FQREIGN PATENTS OR APPLICATIONS 686,24l 5/1964 Canada 75/206 Primary ExaminerBenjamin R. Padgett Assistant ExaminerB. Hunt Attorney, Agent, or Firm-Harold J. Rathbun ABSTRACT 2 Claims, No Drawings METHOD F MAKING LlGHT DUTY V ELECTRICAL CONTACTS COMPOSED or SILVER AND RUTHENIUM DIOXIDE I mixing fine, irregular silver powder with ultra fine ruthenium metal powder to provide a mixture having a fine, evenly dispersed ruthenium content in the range 0.1 to 10.0 atomic percent; compacting the mixture into a desired shape; heating the compacted shape in a suitable atmosphere for a period of time to effect sintering of same; and internally oxidized the sintered compacts to convert the ruthenium metal into, ruthenium dioxide. 1 I

The invention further provides a light-duty electrical contact which is produced by the method outlined in the preceding paragraph.

The foregoing and other features according to the invention will be better understood fromthe-following' description of specific embodiments of the invention.

The electrical contact material, zccordmggm the invention which, as previously stated, is suitable for lightduty applications, consists of a mixture of silver'and ruthenium dioxide, and theconcentration of ruthenium dioxide can vary from 0.1 to 13.0 atomic ercent. The contact material is bestfabricated by powder metallurgical techniques and the preferredand most economical material is a material having a ruthenium dioxide content of 1.3 percent. Vacuum and gas melting techniques are unsuitable because it is not possible to disperse the ruthenium phase finely and evenly throughout the silver.

Thus in a method according to the invention fine, irregular silver powder, and ultra fine ruthenium powder are initimately mixed together such that the ruthenium content of the mixture is in the range 0. 1 to10.0 atomic percent. The intimate mixing can be effectedby dry tumble milling for a period of time of theorder of 2 to 24 hours. Alternatively, the intimate mixing of the powder particles can be effected by dry tumbling in the presence of glass spheres, or by milling underacetone.

The size and shape of the metal powder particles is of prime importance in the manufacture of optimum silver-ruthenium dioxide materials and both powders should preferably be as fine as is economically possible.

This in practice involves the use ofprecipitatedsilv'er' contact material and facilitates the rapid oxidation of silver-ruthenium alloys which are to be internally oxidized to obtain a fine, even dispersion ofruthenium dioxide in the silver;

The powder mixture is then compacted, using molds, into the desired shape for the electrical contacts. The compacting can for example be effected at a pressure of the order of 10 to 20 tons per square inch to give green densities of the order of percent of the theoretical maximum density.

The contact compacts are then sintered by being heated in a neutral or reducing atmosphere, for example 90%N /10%H for a period of time of not less than one hour. The upper temperature limit for the sintering operation is 960.5C., i.e., the melting point of silver. In order to maximize the sintering rate a temperature just below the melting point temperature should be utilized, for example a temperature of the order of 930C. The sintering process increases the density of the contact material to between percent and percent of the theoretical maximum density, the actual density attained being dependent upon the sintering time and temperature.

With silver-ruthenium alloys which have been sinteredin a reducing atmosphere it should be noted that an increase in the dimensions of the contact compact occurs duringthe subsequent internal oxidation step due to an increase in the volume of the material when the ruthenium is converted to ruthenium dioxide, and to the generation of pockets of steam due to the reaction of oxygen with any dissolved/residual hydrogen. Thelatter effect can be avoided by sintering the initial alloy in a neutral atmosphere.

Cold welding of the soft silver powder particles readilyoccurs when compacting is effected at high pressures;forexample40 tons" per square inch, and this welding resultsin pocketsof trapped air in the green contact compact; The pockets of air are expanded dur ing" the sintering stepand can, therefore, cause distortion and even'expansion of the compact. Thus it is important to ensure that high forming pressures of this order are avoided during the method according to the invention;

Thesihte'red contact compacts are then internally oxidized being heated in air at a temperature of the order of 930C for a period of time of not less than 1 hour. This oxidation process completely converts the sub-surface particles of ruthenium metal in the silver (Ag) to ruthenium dioxide (RuO Metal particles at greater depths will only be partially oxidized on their surfaces;

Ruthenium dioxide is a conducting oxide which exhibits very low electrical resistivity and is contained in thecontact material as a fine, even dispersion.

The density of the contact material may then, if desired, be increased to at least percent of the theoretical rnaximum density by a stamping operation at a ressureof the order'of 40 to 45 tons per square inch.

The material of the electrical contacts produced by this method'exh ibits low, stable contact resistance at low contact forces over a period of years under atmospheric conditions which would normally tarnish and corrodeknownsilver base contact materials such as silver-cadmium oxide or silver alone.

in an alternative method according to the invention, the green silver-ruthenium compacts can be compacted at a pressure of the order of tons per square inch, and then sintered in air for a period of the order of 1 hour at a temperature of the order of 930C. This sintering process simultaneously sinters the contact material and oxidizes the ruthenium to RuO The ruthenium powder particles situated well below the surface of the compacts are oxidized and the density of the contact material is increased from 70 to 80 percent of the theoretical maximum density.

As with the previous method, the density of the contact material may then, if desired, be increased to at least 95 percent of the theoretical maximum density by a stamping operation at a pressure of the order of 40 to 45 tons per square inch.

The contact material produced by this alternative method also exhibits low, stable contact resistance at low contact forces for long periods in tarnishing atmospheres.

It should be noted that the simultaneous sintering and oxidation of green silver-ruthenium contacts results in a net shrinkage when the material is initially compacted at 10 tons per square inch. At tons per square inch a net expansion occurs.

When, during the sintering step of the methods according to the invention, the silver recrystallizes and grain growth begins, the grains grow until they meet a ruthenium dioxide particle. The ruthenium dioxide particles impede further grain growth and remain in the grain boundaries to effectively anchor them in position. Thus the ruthenium dioxide content of the contact materials produced by the methods according to the invention is mostly located in the grain boundaries in the silver.

The contact resistance properties of the light-duty electrical contact materials according to the invention in comparison with silver (Ag) and silver-cadmium oxide (AgCdO) contact materials are indicated in the table given below:

CONTACT RESISTANCE (OHMS) The contact resistance is shown as a function of contact force after 21 hours exposure to a moist H 8 atmosphere, i.e., an atmosphere containing 700.0 mm H 8 and 17.0 mm of H 0.

It should be noted that when low tolerance contact dimensions are required it is important to avoid excessive shrinkage of the contact compact during the sintering step of the methods according to the invention. The shrinkage which occurs during the sintering step is directly influenced by the initial forming pressure used to press the green contact compacts and therefore the correct choice of the initial forming pressure is, under these circumstances, very important.

An advantage of the silver-ruthenium dioxide is that it is readily solderable with soft solder, it is readily capable of heading to form a rivet, and it is capable of being brazed.

In a further alternative method according to the invention the ruthenium metal powder utilized in the methods outlined in preceding paragraphs is replaced by ruthenium dioxide powder such that the ruthenium dioxide content of the silver-ruthenium dioxide mixture is in the range 0.1 to 13.0 atomic percent. The silverruthenium dioxide mixture is then compacted and sintered in an inert atmosphere in the manner outlined in preceding paragraphs. While this production method produces a silver-ruthenium dioxide material that may be suitable for certain applications it is not the preferred method because it results in a less favorable dioxide particle size distribution and an inferior dispersion of dioxide particles within the silver matrix.

It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation in its scope.

What is claimed is:

l. A method of producing a light-duty electrical contact including the steps of mixing fine, irregular silver powder with fine ruthenium dioxide powder to provide a mixture having a ruthenium dioxide content in the range 0.1 to 13.0 atomic percent; compacting the mixture into a desired shape; and heating the compacted shape in an inert atmosphere for a period of time to effect sintering of same.

2. A method as claimed in claim 1 wherein the compacting is effected at a pressure in the range 10 to 20 tons per square inch, and wherein the sintering is effected at a temperature in the range 850 to 940C for a period of time of not less than 1 hour. 

1. A METHOD OF PRODUCING A LIGHT-DUTY ELECTRICAL CONTACT INCLUDING THE STEPS OF MIXING FINE, IRREGULAR SILVER POWDER WITH FINE RUTHENIUM DIOXIDE POWDER TO PROVIDE A MIXTURE HAVING A RUTHENIUM DIOXIDE CONENT IN THE RANGE 0.1 TO 13.0 ATOMIC PERCENT; COMPACTING THE MIXTURE INTO A DESIRED SHAPED; AND HEATING THE COMPACTED SHAPE IN AN INERT ATMOSPHERE FOR A PERIOD OF TIME TO EFFECT SINTERING OF SAME.
 2. A method as claimed in claim 1 wherein the compacting is effected at a pressure in the range 10 to 20 tons per square inch, and wherein the sintering is effected at a temperature in the range 850* to 940*C for a period of time of not less than 1 hour. 